1. Field of the Invention
The present invention relates to improved humidification devices for use in continuous positive airway pressure (CPAP) systems. These devices actively humidify air without the need to substantially increase water temperature. The present invention additionally relates to a method for delivering air humidified by non-heat-based humidifiers to airflow delivered to the subject with a CPAP system. The present invention may include a device to collect condensation from the humidified air and recycle the liquid for reutilization in the humidifier.
2. Technical Background
Continuous positive airway pressure (CPAP) devices are used to relieve partial or complete upper airway obstructions in a subject during sleep. A condition known as sleep apnea results when airflow is halted for more than ten seconds during sleep. Sleep apnea leads to decreased blood oxygenation and disrupts sleep. The procedure for administering CPAP treatment has been well documented. An early description can be found in U.S. Pat. No. 4,944,310 (Sullivan). CPAP treatment acts as a pneumatic splint of the airway by applying positive pressure, usually in the range 4 to 20 cm H2O. The air is supplied to the airway by a motor driven blower whose outlet passes via an air delivery hose to a nose and/or mouth mask sealed to a patient's face. An exhaust port is provided in the delivery tube proximate to the mask. CPAP pressure is increased on the detection of pre-defined patterns to provide increased airway pressure to subvert, ideally, the occurrence of the obstructive episodes and the other forms of breathing disorders.
Humidification is an important aspect of the CPAP procedure. The high airflow generated from the CPAP device removes moisture from a subject's nasal cavity, leaving a feeling of dryness and congestion. This dryness is uncomfortable and prevents many users from using the CPAP. In addition to dryness, non-humidified CPAP air may cause bleeding, swelling, excess mucous, congestion, or sneezing. The irritation also creates a very fertile ground for infections. The irritation may be cumulative, building up over time. The only way to reduce the irritation is to add moisture. Humidification therefore can be an important part of CPAP treatment. Besides humidification, water soluble lotions, solutions, or sprays for the nose and prescribed medications such as Nasonex and Flonase can be used to alleviate problems associated with CPAP air.
The prior art includes many references to humidification devices requiring heat (heat-based humidifiers). An example of this is found in U.S. Pat. No. 6,877,510 (Nitta). Heat-based humidifiers, such as heat vaporization humidifiers, heat the liquid as well as the airflow, increasing the maximum amount of water vapor the air can hold. They can also be adjusted to produce more or less moisture by altering the amount of heat applied. Also, the water chamber can be much smaller than in a passive humidifier. An integrated heat-based humidifier, however, cannot be heated as high as a stand-alone heated-based humidifier, due to the close proximity of the heating element to the CPAP. Also, as described below, heat-based humidifiers may produce more condensation than non-heat-based humidifiers, due to a higher temperature difference between the CPAP air and the ambient room temperature. Because of this, these humidifiers are sometimes set at lower constant humidification levels throughout the night, which reduces condensation during the coldest part of the night but prevents optimal humidification at the start and end of the night when temperatures are higher. Other main drawbacks of heat-based humidifiers are that they consume much or more electric power because of the high amount of heat needed to operate, and they require more time to begin humidification than non-heat-based humidifiers because of the need to substantially heat the humidifying liquid. Also, microbial growth is greater in heat-based humidifiers, increasing the risk of patient exposure to, for example, bacteria, yeasts, and molds. Finally, the components of heat-based humidifiers may have to be replaced more often than in non-heat-based humidifiers, as steam canisters need to be replaced every so often and can usually only be purchased from the original manufacturer of the steam humidifier. This increases time and costs associated with maintaining heat-based humidifiers as opposed to non-heat-based humidifiers.
The prior art also describes passive or “passover” humidifiers, which do not require heat. An example of this as integrated into a CPAP device is shown in U.S. Pat. No. 6,827,340 (Austin). These humidifiers are quite simple and, for the most part, self-regulating. They rely on the fact that an air stream passing over a reservoir of liquid or past a wick saturated with that liquid will pick up whatever moisture it can as it “passes over” the liquid. The higher the relative humidity, the harder it is for the air stream to pick up moisture, which is why these humidifiers are self-regulating (as humidity increases the humidifier's water-vapor output naturally decreases due to the decreasing difference in vapor pressure). Although these humidifiers are simple and do not require a heat source, there is no way to increase or decrease the amount of air humidification should this level be too low or high. Also, when integrated into a CPAP device, the surface area of the water used to humidify the air is necessarily smaller, resulting in lower humidification levels. As a result, this humidifier is only feasible in CPAPs set at lower-end pressures, as higher-end pressures will not produce adequate humidification levels. Conversely, increasing the surface area of the water contacting the air will increase the size of the humidifier, to the point where it would be difficult to integrate it into the CPAP. In these cases, the humidifier must be a separate attachment, not part of the CPAP system itself. Also, because of the larger size, these humidifiers may suffer from fill and spill problems because of the large size of the reservoir tank.
Condensation is a problem for any humidifier in a CPAP system. Because the greater the temperature difference between the ambient room temperature and the CPAP air the more condensation is produced, heat-based humidifiers are more susceptible to condensation than other humidifiers, since the air they produce is hotter than air produced in “cold” humidifiers. This is especially a problem at night, when the ambient temperature usually decreases in relation to the temperature of the humidifier. Condensation produces an accumulation of water in the CPAP tubing. This water produces a disruptive gurgling noise and added resistance to the CPAP circuit that results in large, transient fluctuations in mask pressure. Also, as little as 10 ml of condensate can cause an inspiratory pressure drop of up to 5.6 cm H2O. Thus, preventing the formation of condensate in the CPAP tubing is vital to ensuring CPAP therapy remains effective and tolerable. Some CPAP devices with heat-based humidifiers use heated CPAP tubing to prevent condensation, but this can be dangerous. See U.S. Pat. No. 5,537,996 (McPhee). Others use sensors which detect the ambient temperature and adjust heat output accordingly, so that the temperature of the CPAP air is never substantially greater then the temperature of the ambient air, minimizing condensation. See U.S. Pat. No. 5,558,084 (Daniell).
It is an object of the present invention to avoid the drawbacks of heat-based humidifiers and passive humidifiers in CPAP systems. It is further an object of the present invention to produce an integrated, compact, adjustable, cost-effective humidifier for use in CPAP systems. It is even further an object of the present invention to be less susceptible to contaminant growth because it operates at lower temperatures, creating a less hospitable environment for bacteria and other microbes than in heat-based humidifiers. Finally, it is even further an object of the present invention to produce less condensation by operating at temperatures much closer to ambient room temperatures, and through the use of new condensation-removal features in the CPAP device.